Explaining rates

On page 88, you saw that magnesium and dilute hydrochloric add react together  

The particles in the liquid move around continually. Here an add particle is about to collide with a magnesium atom. 

If the collision has enough energy, reaction takes place. Nfognesium chloride and hydrogen are formed. 

If there are lots of successful collisions in a given minute, then a lot of hydrogen is produced in that minute. In other words, the reaction goes quickly—its rate is higjh. If there are not many, its rate is low. The rate of a reaction depends on how many successful collisions there are in a given unit of time. 

Why rate increases with concentration If the concentration of the add is increased, the reaction goes faster. It is easy to see why  

---

A simple unifying theory to explain rate and specificity in atom abstraction reactions has yet to be developed. However, as with addition reactions, it is possible to devise a set of guidelines to predict qualitatively the rate and outcome of radical transfer processes. The following are based on those suggested by Tedder 2... 

Most of the characteristics invoked to explain rate accelerations and rate retardations by micelles are valid for vesicles as well. For example, the alkaline hydrolysis of A-methyl-A-nitroso-p-toluenesulfonamide is accelerated by cationic vesicles (dioctade-cyldimethylammonium chloride). This rate acceleration is the result of a higher local OH concentration which more than compensates for the decreased polarity of the vesicular pseudophase (compared to both water and micelles) resulting in a lower local second-order rate constant. Similar to effects found for micelles, the partial dehydration of OH and the lower local polarity are considered to contribute significantly to the catalysis of the Kemp elimination " by DODAB vesicles. Even the different... 

The mechanism of Ziegler-type polymerization has not only to explain rate expressions found by kinetic measurements, but also the structure of the polymer. The structure and the molecular-weight distribution of the polymers are a record of what happened during the polymerization reaction. What is to be explained may be summarized by discussing propylene as a monomer. 

The oscillatory kinetics of CO oxidation over Pt single crystals has been studied at atmospheric pressure by Yeates et al. (95). They present a model to explain rate oscillations that relies on the oscillatory formation of surface platinum oxide, which was observed in postreaction analysis and was related to the presence of silicon impurity. 

Stanton CL, Kuo I-FW, Monday CJ, Laino T, Houk KN (2007) QM/MM metadynamics study of the direct decarboxylation mechanism for orotidine-5 -monophosphage decarboxylase using two different QM regions acceleration too small to explain rate of enzyme catalysis. J Phys Chem B 111 12573-12581... 

Reaction mechanisms may be developed to explain rate laws, but often they are developed in parallel, one helping the other. In translating a mechanism into a rate law a useful tool is Bodenstein s steady state approximation (SSA) or stationary state hypothesis. This approximation assumes that after a very short interval of time any reactive intermediate, which because of its reactivity will only be present in negligible proportions, will have its rate of decay equal to its rate of production, i.e., it will reach a steady concentration on a vanishingly small time scale. If this did not happen the amount of the intermediate would build up to measureable proportions and it would become an intermediate product. It is assumed at any instant that dcj /dt = 0, where R is the reactive intermediate. 

The proposed mechanism involves a rapid pre-equilibrium formation of [MoOClJ followed by replacement of a second chloride by nitrate to explain rate inhibition by C1- ... 

The selection rules for electrocyclic ring-openings can also explain rate differences in the solvolysis of cyclopropyl halides as a function of stereochemistry. For example, shown in... 

The concept of a transition state originates from transition-state theory. Before transition-state theory, chemists had explained rates of reactions in terms of collision theory, which is based on the kinetic theory of gases. It treats collisions by regarding the reacting molecules as hard spheres colliding with one another. Transition-state theory does not conflict with collision theory. It assumes that reactions involve collisions but takes into account some of the details of the collision, such as how the reacting molecules must approach one another for a reaction to be possible and what the effect of a solvent might be. None of these factors are taken into account by simple collision theory. 

The rate reduction effects can be calculated by the influence of the pressure on the Gibbs free energy of the equilibria. The calculations show that even though there may be some effects of this factor if the overpressure in the pores varies as much as 100 bar, it certainly cannot explain rate variations of a factor as high as 2. 

---